The present invention relates generally to nozzles for dispensing fluid, and more particularly to a hybrid atomizing fuel nozzle for gas turbine applications.
A typical gas turbine combustor includes an array of fuel injectors disposed within a combustion chamber. A compressor provides pressurized air past the fuel injectors. Atomized fuel is sprayed from the injectors and the air facilitates ignition of the fuel downstream in the chamber.
One particularly useful nozzle for the fuel injectors is an airblast nozzle. In conventional airblast nozzles, fuel is directed through a fuel annulus and dispensed out through an annular fuel orifice between an inner airflow directed centrally through the nozzle, and an outer air flow directed inwardly through one or more in-flow swirlers. The fuel impinges upon an annular, tapered prefilmer at the downstream end of the fuel annulus, and is then atomized by the inner and outer air flows.
Airblast atomizers have significant advantages in their application to gas turbine combustion systems. The fuel distribution is primarily dictated by the air flow patterns, and hence the combustor outlet temperature is fairly insensitive to changes in fuel flow. Combustion is characterized by the absence of soot formation, resulting in relatively cool liners walls and minimum exhaust smoke. The primary disadvantages of airblast atomizers are their rather narrow stability limit and poor atomization quality at start up, owning to the low velocity of air through the combustor.
The drawbacks of airblast atomizers can be substantially overcome by combining the airblast atomizer with a pressure swirl atomizer, in what is generally referred to as a xe2x80x9chybrid airblast atomizerxe2x80x9d. A pressure swirl atomizer typically includes a swirl chamber that directs swirling fuel through a central orifice, where the fuel quickly atomizes into a thin, conical spray. Pressure swirl atomizers are known for their easy light-up and wide stability limits, although they are somewhat more sensitive to changes in fuel flow than pure airblast nozzles. At low fuel flows in a hybrid airblast atomizer, all of the fuel is supplied from the pressure swirl atomizer, and a well-atomized spray is obtained to give sufficient combustion at start-up and idle running conditions. Under cruise and higher power conditions, the fuel is supplied to both the pressure swirl atomizer and the airblast atomizer. At maximum power, most or all of the fuel is supplied to the airblast atomizer. By this structure, the merits of the pressure swirl atomizer at lower fuel flows are combined with the virtues of airblast atomization at higher flow rates.
While it is accepted that the hybrid airblast fuel injection strategy results in better engine performance throughout the engine cycle, current designs are believed deficient, in that the fuel circuit and the inner air flow for the airblast portion of the nozzle are typically radially crisscrossed. That is, the fuel passage for the pressure swirl atomizer is typically ported down the center of the housing stem. The fuel passage for the airblast atomizer is radially-outwardly spaced from (or surrounds) the pressure swirl atomizer fuel passage. At the discharge end of the nozzle, the fuel passage for the airblast atomizer is connected to a fuel annulus, which extends to an annular fuel discharge opening at the tip of the nozzle. Combustion air is ported radially inward of the airblast fuel passage (and outward of the pressure swirl atomizer fuel passage) near the tip of the nozzle to provide the inner air flow. The porting of the air through the nozzle tip is typically accomplished by drilling or forming multiple, radially-directed passages in the nozzle tip. The air passages cross radially through either the fuel annulus for the airblast fuel conduit or the fuel passage just prior to the annulus. The air passages extend to an annular air discharge opening radially inward of the fuel discharge opening.
For good secondary atomization, it is important that there be a sufficient number of air passages and that the air passages have a sufficient size to provide adequate air flow to the inner air circuit. The nozzle stem must also have sufficient size and strength to support the nozzle tip, as well as to maintain fuel circuit integrity. If the relatively cold fuel (at approximately 300xc2x0 F.), passes too closely to the hot air passages (which often reach temperatures of 1000xc2x0 F. at maximum engine power), rapid heating of the fuel can occur, which increases the fuel""s propensity for coking. High differential temperatures and high thermal gradients can also lead to relatively high thermal stresses in the nozzle. These stresses can limit the working life of the fuel nozzle.
On the other hand, the size and weight of the nozzle is an important factor, as the size and weight of the nozzle can effect the size and weight of the engine, and hence the fuel economy of the aircraft. Aircraft manufacturers have demanded smaller and lighter-weight nozzles in an attempt to improve the fuel economy of the aircraft. In some instances this has required compromises in the number and size of the air passages through the nozzle stem. This can reduce the efficiency of the nozzle, which is undesirable in certain instances. This has also required the use of high strength, expensive allow materials for the nozzle tip, and complex heatshielding of the nozzle and the housing stem, which increases the manufacturing cost of the nozzles.
As such, while the known hybrid airblast nozzles overcome some of the disadvantages of simple airblast nozzles, it is believed there is a demand in the industry for a further improved hybrid airblast nozzle, and in particular a new and improved hybrid airblast nozzle that has a small, lightweight package; has simple air and fuel flow passages; requires less heatshielding; does not require costly, high-strength alloy materials; provides longer operating life as a result of reduced coking and thermal stresses, and which is thereby simpler and less costly to manufacture.
The present invention provides a novel and unique nozzle for a fuel injector, and more particularly a hybrid atomizing fuel nozzle for gas turbine applications. The fuel nozzle has a small, lightweight package; has simple air and fuel flow passages; requires less heatshielding; has long operating life; and which thereby is simpler and less costly to manufacture. It is also relatively easy and straightforward to tailor the air flow through the nozzle for a particular application, without substantial modifications and without increasing the size of the nozzle.
According to the present invention, the inner air flow for the airblast portion of the nozzle is provided downstream from and entirely exterior to the nozzle tip. The inner air flow is provided by a radial in-flow swirler surrounding the nozzle tip, which directs the combustion air radially inward toward the tip. The fuel in the airblast fuel circuit is directed radially outward, preferably in discrete streams from a plurality of fuel passages formed around the tip of the nozzle, toward a prefilmer surface downstream of the radial in-flow swirler. The inner air flow passes radially inward through the fuel flow, downstream of the nozzle tip. By directing the air through the fuel stream, the fuel is spread out evenly in a thin film across the prefilmer surface, which results in an even, fully-atomized spray leaving the prefilmer surface.
The manufacturing costs of the nozzle are also reduced, as there is no need for drilling or forming air flow passages in the nozzle tip. The size of the nozzle can thereby be reduced, which reduces the weight (and cost) of the nozzle. The heatshielding requirements of the nozzle tip are also somewhat lessened as the high temperature combustion air is kept away from the sensitive portions of the nozzle tip. The operational life of the nozzle is also increased as the coking and thermal stresses are reduced. The air flow through the radial inflow swirler can also be easily adjusted merely by changing the length of the vanes and passages in the in-flow swirler, without having to otherwise modify the nozzle tip.
The hybrid fuel nozzle of the present invention is fixed to the end of a nozzle stem. The nozzle stem includes a primary/pilot fuel passage extending centrally through the stem, and a secondary/main fuel passage surrounding the primary fuel passage. The primary/pilot fuel passage is fluidly connected to a primary/pilot nozzle portion including a pressure swirl atomizer that provides fuel in a primary atomized spray. The secondary/main fuel passage is fluidly connected to a secondary/main nozzle portion, radially outward surrounding the primary/pilot nozzle portion, and including an airblast nozzle that provides fuel in a secondary atomized spray.
A shroud and swirler assembly surrounds the secondary nozzle portion and is fixed to the downstream end of the stem. The shroud and swirler assembly includes an annular body with the radial in-flow swirler at the upstream end of the body. The radial in-flow swirler directs air radially inward toward the secondary nozzle portion in a swirling, inner air flow. The internal prefilmer surface is located downstream from the radial in-flow swirler, and preferably has an annular, convex (outwardly-diverging) configuration. An outer air swirler, surrounding the annular body, provides a second swirling air flow downstream of the annular body. The outer air swirler has an inwardly-directed forward end, which directs the air in a converging manner in a swirling, outer air flow.
The secondary nozzle portion includes at least one fuel passage, and preferably includes a plurality of discrete fuel passages in an annular configuration, which direct fuel in streams radially outward toward and against the prefilmer surface. The fuel passages can be angled with respect to the central axis of the nozzle such that the passages supply the fuel in a swirling manner radially outward and downstream from the secondary nozzle portion. The fuel streams are directed outward through the swirling air from the radial in-flow swirler, which causes the fuel to accelerate and evenly distribute across the prefilmer surface in a thin, continuous sheet. The inner and outer air flows then cause the thin fuel sheet to quickly atomize and form a conical spray pattern downstream of the prefilmer surface.
An annular sleeve is disposed between the secondary nozzle portion and the annular body of the shroud and swirler assembly, and includes a forward (downstream) end with an annular concave surface. The concave surface of the sleeve is located adjacent the radial inflow swirler, and directs the air flow directly against the secondary nozzle portion, such that the high temperature combustion air does not contact the nozzle stem, or any other portion of the nozzle upstream from the fuel passages in the secondary nozzle portion. This reduces the heatshielding requirements around the nozzle tip.
The entire air circuit for the nozzle is integrated into one subassembly, while the entire fuel circuit of the nozzle is integrated into a separate subassembly. The subassemblies can be easily fixed (e.g., welded or brazed) together during assembly of the fuel injector and removed for inspection and repair or replacement. This also reduces the manufacturing and maintenance costs of the injector.
A hybrid atomizing fuel nozzle is thereby provided that has a small, lightweight package; has simple air and fuel flow passages; has long operating life; requires less heat-shielding; and which is thereby simpler and less costly to manufacture. It is also easy to obtain greater air flow for better atomization, or to otherwise adjust the air flow as necessary for a particular application.
Further features of the present invention will become apparent to those skilled in the art upon reviewing the following specification and attached drawings.